Essentials of Neutral Grounding System
Indian Standard IS: 3043 – 2018 [Code of Practice for Earthing – Second Revision] says the following: Earthing refers to the complications which arise when electricity conducts via the ground. The terms earth and earthing have been used here regardless of whether or not the earth is employed as a low impedance fault current return path. Earth is now employed for regulating the voltage of system neutrals rather than as part of the return circuit. The earth connection ensures service continuity and protects equipment from damage as well as human life. This article states different opinions & analyses related to neutral grounding in electrical systems.
IS:3043 further states that “The purpose of an earthing (grounding) system is to provide as nearly as possible a uniform potential and as close to zero or absolute earth potential under and around a station. So that all parts of apparatus other than live parts are at earth potential. Including operators and attendants.
Providing such an earth surface of uniform potential under and surrounding the station, there can exist no difference in a short distance big enough to shock or injure an attendant when short-circuits or other unusual phenomena occur. The Code suggestions are made for these goals to be realized.
System Earthing Vs Equipment Earthing:
Earthing associated with a current-carrying conductor is essential to the security of the system and is known as system earthing. Earthing of non-current carrying metalwork and conductor is known as equipment earthing and is essential to the safety of human life, animals, and property.
What is Grounding in an electrical system?
The term grounding in electrical can refer to both - equipment grounding and system grounding. The connection of non-current carrying conductive objects such as conduit, cable trays, enclosures, and motor frames to earth ground is called equipment grounding. System grounding in electrical refers to the solid or current-limiting connection of the neutral points of current-carrying conductors, such as the neutral point of a circuit, a transformer, spinning machinery, or a system, to earth/ground.
What is an Ungrounded System?
The systems were little back then, and they were not interconnected to form a grid. The most conspicuous fault was an earth-fault that, due to its modest size, was frequently self-extinguished. Because service continuity was not jeopardised and the ungrounded neutral idea was adopted.
As the size of electrical systems developed in terms of enormous transmission line networks, steady-state and transient overvoltage began to occur on the transmission lines. The voltage of the phases that were not affected by the problem increased by 1.73 times. This began to put a strain on the insulation, eventually causing it to fail. In the delta system, a line to ground voltage surged over 2.5 times the typical voltage when neutral instability has occurred.
In a healthy system, the neutral voltage to the ground is zero. When a ground fault occurs, the neutral voltage rises in proportion to the severity of the fault. As a result, the neutral becomes unstable as its voltage varies concerning the ground.
Natural disturbances such as lightning, high-speed winds, earthquakes, and other factors such as trees falling on overhead lines or support structures generate power system faults. Faults in an industrial power system have been observed to occur in the following range.
Charging currents in LV systems range from 0.1 to 2A, up to 10A in 13.8kV systems. A typical range is shown in the table below:
Multiple Ground Faults:
On ungrounded neutral systems, this can happen. While a ground fault on one phase of an ungrounded system does not produce an outage because the fault-free phases have line-to-line voltage impressed on their line-to-ground insulation. The longer the first ground fault lingers, the more likely a second ground fault will occur on another phase.
Insulation has overstressed by up to 73%. In addition, the system is degrading at a faster pace. Due to the cumulative overvoltages imposed on it by successive ground faults that have occurred over several years.
Classification of Various Grounding in Electrical Systems:
Solidly Grounded - When the neutral of every generator or transformer bank in that area is connected solidly and permanently to the earth through a solidly grounded connection, the station is called adequately grounded. When all of the system's critical producing or transmission stations are adequately grounded, the system is deemed well-grounded.
Resistance Grounded - When the neutral of every generator or transformer bank in that section is permanently connected to the earth through a low resistance connection, the station is said to be resistance grounded. When all of a system's main generating stations or transmission stations are connected by a low-resistance connection, the system is resistance grounded.
Reactance Grounded - When an air- or iron-core reactor is introduced between the neutral and the earth, the system is deemed reactance grounded. This holds true whether the system is grounded via a grounding transformer or simply a portion of the generator or transformer capacity is reactance grounded at any given moment.
Tune Reactance Grounded - When one conductor is grounded and the aggregate reactive current passed by the neutral reactors or their equivalent substantially equals the aggregate charging current to the ground of the system, the system is considered tuned reactance grounded, just like the Petersen Coil, Bauch Transformer, and Dissonance Coil.
Ungrounded - A system is considered ungrounded if there are no conductive paths between the neutral and the earth other than through a potential transformer. There are chances of a heavy short circuit or dead short circuit for such arrangement if proper grounding is not provided.
Concept of Effective Grounding:
The degree of system grounding stated in IEEE142 determines the effectiveness of a solidly grounded system. The following are the two most important requirements:
1.- The line to ground fault ILG at least 60% of the permissible three-phase fault current I3ϕ
2.- The second criterion is that the un-faulted phase voltage to ground (VUNF) must be at least 80% of the systems rated line to voltage (VLL).
This blog has documented some of the fascinating historical opinions and the slow but steady growth of the foundation concepts up to today's practice (and standards). Resistance grounding, reactance grounding, and tuned reactor grounding in electrical systems are in addition to successful grounding analyses.
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